Resistance variable memory elements, which include electrokinetic memory elements using chalcogenides, have been investigated for suitability as semi-volatile and non-volatile random access memory devices. A representative chalcogenide resistance variable memory element is disclosed in U.S. Pat. No. 6,348,365 to Moore and Gilton.
In one type of chalcogenide resistance variable memory element, a conductive material, for example, silver and copper, is incorporated into a chalcogenide glass. The resistance of the chalcogenide glass can be programmed to stable higher resistance and lower resistance states. An unprogrammed chalcogenide variable resistance element is normally in a higher resistance state. A write operation programs the element to a lower resistance state by applying a voltage potential across the chalcogenide glass and forming a conductive pathway. The element may then be read by applying a voltage pulse of a lesser magnitude than required to program it; the resistance across the memory device is then sensed as higher or lower to define two logic states.
The programmed lower resistance state of a chalcogenide variable resistance element can remain intact for an indefinite period, typically ranging from hours to weeks, after the voltage potentials are removed; however, some refreshing may be useful. The element can be returned to its higher resistance state by applying a reverse voltage potential of about the same order of magnitude as used to write the device to the lower resistance state. Again, the higher resistance state is maintained in a semi- or non-volatile manner once the voltage potential is removed. In this way, such an element can function as a semi- or non-volatile variable resistance memory having at least two resistance states, which can define two respective logic states, i.e., at least a bit of data.
One exemplary chalcogenide resistance variable device uses a germanium selenide (i.e., GexSe100−x) chalcogenide glass as a backbone between first and second electrodes. The germanium selenide glass has, in the prior art, incorporated silver (Ag) and silver selenide (Ag2+/−xSe) layers in the memory element. The element is programmed by applying a sufficient voltage across the electrodes to cause the formation of a conductive path between the two electrodes, by virtue of a conductor (i.e., such as silver) that is present in metal ion laced glass layer.
It would be desirable to have a structure and method for adjusting the programming voltages used for a memory element.